Laminated sheet, method of producing the sheet, exhaust gas processing device, and method of producing the device

ABSTRACT

A disclosed laminated sheet includes a first mat containing first inorganic fibers and a second mat containing second inorganic fibers, the second mat being laminated on the first mat. The average fiber length of the first inorganic fibers is larger than the average fiber length of the second inorganic fibers. Development of cracks in the outer surface of the laminated sheet can be prevented by winding the laminated sheet around an exhaust gas processing unit such that the first mat faces outward.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a laminated sheet, a methodof producing the laminated sheet, an exhaust gas processing deviceincluding the laminated sheet as a holding sealer, and a method ofproducing the exhaust gas processing device.

2. Description of the Related Art

The number of cars in the world has greatly increased since thebeginning of this century, and along with the increase in the number ofcars, the amount of exhaust gas from internal combustion engines of carshas greatly increased. Especially, exhaust gas from diesel enginescontains various pollutants and is a serious threat to the worldenvironment.

To reduce pollution resulting from exhaust gas, various exhaust gasprocessing devices have been proposed and put into practical use.Normally, an exhaust gas processing device is provided in the path of anexhaust pipe connected to an exhaust gas manifold of an engine, andincludes a casing composed, for example, of metal and an exhaust gasprocessing unit disposed in the casing and having a large number ofcells separated by cell walls. In many cases, these cells are arrangedto form a honeycomb structure, and in such cases, the exhaust gasprocessing unit is simply called a “honeycomb structure”. Examples ofexhaust gas processing units include catalyst carriers and exhaust gasfilters such as a diesel particulate filter (DPF). Taking a DPF as anexample, when exhaust gas passes through the cells of a honeycombstructure (or an exhaust gas processing unit), fine particles(particulates) are trapped by the cell walls and removed from theexhaust gas. Examples of materials for an exhaust gas processing unitinclude metals, alloys, and ceramics. Honeycomb filters composed ofcordierite are popular as ceramic exhaust gas processing units. In thesedays, in terms of heat resistance, mechanical strength, and chemicalstability, porous silicon carbide sintered bodies are widely used asmaterials for exhaust gas processing units.

Normally, a holding sealer is provided between an exhaust gas processingunit and a casing. A holding sealer prevents an exhaust gas processingunit from hitting the inner side of a casing when, for example, anautomobile is running and thereby prevents damage to the exhaust gasprocessing unit. Also, a holding sealer prevents leakage of exhaust gasthrough a gap between an exhaust gas processing unit and a casing.Further, a holding sealer prevents an exhaust gas processing unit fromfalling off due to exhaust gas pressure. Meanwhile, to maintainreactivity of an exhaust gas processing unit, it is necessary to keepthe exhaust gas processing unit at high temperature. For this purpose, aholding sealer must also have good thermal insulation performance.Holding sealers composed of inorganic fibers, such as alumina fibers,are known to satisfy requirements as described above.

An exemplary holding sealer composed of inorganic fibers is wound aroundat least a portion, other than the openings, of the outer surface of anexhaust gas processing unit, with two ends of the holding sealer beingjoined together. The holding sealer is fixed with tape to the exhaustgas processing unit. The exhaust gas processing unit wrapped by theholding sealer is inserted into a casing to produce an exhaust gasprocessing device.

Recent internal combustion engines emit exhaust gases at hightemperatures and pressures. For this reason and to achieve the goalsmentioned below, it is necessary to improve the thermal insulationperformance of holding sealers.

(i) To prevent creation of a gap between a holding sealer and a casingwhich is caused by expansion of the casing due to heat transferred fromthe exhaust gas processing unit through the holding sealer.

(ii) To prevent degradation due to heat of parts (for example,instruments) attached to the outer surface of a casing.

(iii) To improve efficiency of a recycling process for certain types ofexhaust gas processing units such as DPFs (in a recycling process,trapped particulates are combusted at high temperature to enable reuseof exhaust gas processing units).

One way to achieve the above goals is to increase the gap between anexhaust gas processing unit and a casing and increase the thickness of aholding sealer to improve its thermal insulation performance. However,as the thickness of a holding sealer increases, the difference betweenthe outer circumference and the inner circumference of the holdingsealer, when wound around an exhaust gas processing unit, increases. Alarger difference between outer and inner circumferences increases thepossibility of developing cracks in the outer surface (that is oppositeto a surface in contact with the exhaust gas processing unit) of aholding sealer. Such cracks in turn lead to leaks of unprocessed exhaustgas.

Patent document 1 discloses an exhaust gas processing device includingan exhaust gas processing unit and a holding sealer. Multiple groovesare formed on a surface (inner surface), which is in contact with theexhaust gas processing unit, of the holding sealer to preventdevelopment of cracks resulting from the difference between outer andinner circumferences. The grooves formed on the holding sealer reducethe influence of the difference between outer and inner circumferencesand thereby prevent development of cracks as well as resulting leaks ofunprocessed exhaust gas.

[Patent document 1] Japanese Patent No. 3072281

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a laminated sheetincludes a first mat containing first inorganic fibers; and a second matcontaining second inorganic fibers, the second mat being laminated onthe first mat; wherein average fiber length of the first inorganicfibers is larger than the average fiber length of the second inorganicfibers.

The first inorganic fibers and the second inorganic fiber may be made ofthe same material or different materials.

The average fiber length of the first inorganic fibers is preferablywithin a range between about 20 mm and about 120 mm and the averagefiber length of the second inorganic fibers is preferably within a rangebetween about 0.5 mm and about 10 mm.

The first mat and/or the second mat may further contain a binder.

The laminated sheet of an embodiment of the present invention may alsoinclude an interface layer at the interface between the first mat andthe second mat. The thickness of the interface layer is preferablywithin a range between about 0.05 mm and about 2 mm.

The thickness of the laminated sheet is preferably within a rangebetween about 6 mm and about 20 mm.

According to another embodiment of the present invention, a method ofproducing a laminated sheet includes a first step of preparing a firstmat containing first inorganic fibers; and a second step of laminating asecond mat containing second inorganic fibers on the first mat; whereinan average fiber length of the first inorganic fibers is larger than anaverage fiber length of the second inorganic fibers.

The second step may include a substep of forming the second mat directlyon the first mat.

The second step may include substeps of separately preparing the secondmat and then laminating the first mat and the second mat. In this case,the second step may further include a substep of joining the first matand the second mat by adhesive bonding and/or by sewing.

The first mat may be prepared by a needling method.

The second mat may be prepared by a sheet making method.

According to another embodiment of the present invention, an exhaust gasprocessing device includes an exhaust gas processing unit; and a holdingsealer wound around at least a portion of the outer surface of theexhaust gas processing unit; wherein the holding sealer is composed ofthe laminated sheet as described above and wound around the exhaust gasprocessing unit such that the first mat of the laminated sheet facesoutward.

With this configuration, development of cracks in the outer surface ofthe holding sealer can be effectively prevented because the first matforming the outer surface of the holding sealer is composed of inorganicfibers having a larger average fiber length, i.e. a higher tensilestrength, than that of the inorganic fibers of the second mat.

A catalyst carrier or an exhaust gas filter may be used as the exhaustgas processing unit of the above exhaust gas processing device.

Still another embodiment of the present invention provides a method ofproducing an exhaust gas processing device including an exhaust gasprocessing unit and a holding sealer wound around at least a portion ofan outer surface of the exhaust gas processing unit. This methodincludes the steps of preparing the holding sealer with the laminatedsheet produced as described above and winding the holding sealer aroundthe exhaust gas processing unit such that the first mat of the laminatedsheet faces outward.

Also in this case, a catalyst carrier or an exhaust gas filter may beused as the exhaust gas processing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating an exemplary laminated sheet accordingto an embodiment of the present invention;

FIG. 2 is a drawing used to describe assembly of the exemplary laminatedsheet, an exhaust gas processing unit, and a casing to form an exhaustgas processing device;

FIG. 3 is a SEM photograph used to measure the average fiber length ofinorganic fibers;

FIG. 4 is a drawing illustrating an exemplary configuration of anexhaust gas processing device according to an embodiment of the presentinvention;

FIG. 5 is a flowchart showing an exemplary process of producing alaminated sheet according to an embodiment of the present invention;

FIG. 6 is a cut-away side view of a laminated sheet produced by a directlamination method according to an embodiment of the present invention;

FIG. 7 is a flowchart showing an exemplary process of producing anexhaust gas processing device according to an embodiment of the presentinvention;

FIG. 8 is a drawing illustrating a process of fitting an exhaust gasprocessing unit wrapped by a holding sealer into a casing by a press-fitmethod;

FIG. 9 is a drawing illustrating a process of fitting an exhaust gasprocessing unit wrapped by a holding sealer in a casing by a clamshellmethod;

FIG. 10 is a drawing illustrating a process of fitting an exhaust gasprocessing unit wrapped by a holding sealer in a casing by awinding/fastening method;

FIG. 11 is a drawing illustrating a process of fitting an exhaust gasprocessing unit wrapped by a holding sealer in a casing by a sizingmethod;

FIG. 12 is a photograph of a laminated sheet of example 1 used in awinding test; and

FIG. 13 is a photograph of a single-layer sheet of comparative example 1used in a winding test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described below withreference to the accompanying drawings.

FIG. 1 is a drawing illustrating an exemplary laminated sheet accordingto an embodiment of the present invention. The shape of a laminatedsheet according to an embodiment of the present invention is not limitedto that shown in FIG. 1. FIG. 2 is a drawing used to describe assemblyof the exemplary laminated sheet, an exhaust gas processing unit, and acasing to form an exhaust gas processing device.

A laminated sheet 30 of FIG. 1 is used as a holding sealer 24 of anexhaust gas processing device 10 of FIG. 2 and is designed to be woundaround an exhaust gas processing unit 20 of FIG. 2 that is, for example,a catalyst carrier. As shown in FIG. 1, the laminated sheet 30 has aprotrusion 50 and a recess 60 at corresponding ends 70 and 71 that areorthogonal to the direction (X direction) in which the laminated sheet30 is to be wound. After being wound around the exhaust gas processingunit 20, the protrusion 50 and the recess 60 of the laminated sheet 30are fit together as shown in FIG. 2 and the laminated sheet 30 isthereby fixed to the exhaust gas processing unit 20. The exhaust gasprocessing unit 20 wrapped by the laminated sheet 30 is, for example,press-fit into a casing 12 made of a metal cylinder to form the exhaustgas processing device 10. The laminated sheet 30 is mainly composed ofinorganic fibers and may also include a binder as described later.

The laminated sheet 30 is produced by laminating at least two types ofmats composed of inorganic fibers. The average fiber length of theinorganic fibers is different from one mat to another. For example, thelaminated sheet 30 shown in FIG. 1 is produced by laminating a first mat82 and a second mat 84. The average fiber length of inorganic fibersconstituting the first mat 82 is larger than that of the second mat 84.Hereafter, the first mat 82 and the second mat 84 are also called thelong-fiber mat 82 and the short-fiber mat 84, respectively.

Exemplary characteristics of the laminated sheet 30 are described below.

In the case of a conventional sheet (a holding sealer), when it is woundaround an exhaust gas processing unit, tensile stress is applied to theouter surface of the conventional sheet because of a difference L(LO−LI) between an outer circumference LO and an inner circumference LIof the conventional sheet. The influence of the difference L increasesas the thickness of the conventional sheet increases. Therefore,increasing the thickness of a conventional sheet increases thepossibility of developing cracks in its outer surface when it is woundaround an exhaust gas processing unit. If cracks are formed in a surfaceof a sheet, unprocessed exhaust gas may leak through the cracks out ofthe exhaust gas processing device without going through the exhaust gasprocessing unit. One way to obviate this problem is to form multiplegrooves, which are orthogonal to the winding direction, on the innersurface of a sheet (a surface that touches an exhaust gas processingunit when the sheet is wound around) so that the influence of thedifference L is reduced. However, with this conventional method, it isnecessary to change the sizes and shapes of grooves on a sheet to suitthe shapes and dimensions of an exhaust gas processing unit to be used.For example, the width of and the pitch between the grooves to be formedon the inner surface of a sheet are determined such that the total widthof grooves (width x number of grooves) becomes equal to the differenceL. Also, with the conventional method, it is necessary to adjust thedepth of the grooves according to the thickness of a sheet. Thus, theconventional method greatly reduces the efficiency in producing sheets.

Meanwhile, the laminated sheet 30 of this embodiment is made up of thelong-fiber mat 82 and the short-fiber mat 84. Normally, the tensilestrength of a mat composed of inorganic fibers having a larger averagefiber length is higher than that of a mat composed of inorganic fibershaving a smaller average fiber length. In this embodiment, since thelaminated sheet 30 is wound around the exhaust gas processing unit 20such that the long-fiber mat 82 faces outward, development of cracks dueto the difference between outer and inner circumferences can beeffectively prevented. Also, unlike a conventional sheet having groovesfor reducing the influence of the difference between outer and innercircumferences, the laminated sheet 30 according to an embodiment of thepresent invention can be used for exhaust gas processing units havingdifferent shapes and dimensions without changing its configuration andtherefore can be produced efficiently.

Thus, the above embodiment of the present invention makes it possible toprevent development of cracks, which may be caused by the differencebetween the outer and inner circumferences, in the outer surface of thelaminated sheet 30 wound around the exhaust gas processing unit 20.Accordingly, using the laminated sheet 30 as a holding sealer of anexhaust gas processing device makes it possible to effectively preventleaks of unprocessed exhaust gas.

In this embodiment, the average fiber lengths of inorganic fibers in theshort-fiber mat 84 and the long-fiber mat 82 were measured as describedbelow. To measure the average fiber length of the short-fiber mat 84,sample mats (each having a size of 10 cm×10 cm) were prepared and tenareas on each mat were randomly selected. Fibers were sampled from theten areas and the sampled fibers were photographed using a scanningelectron microscope (SEM) at 50× magnification. The lengths of at least50 fibers were measured for each of the ten areas. Then, the measuredlengths of fibers from all of the ten areas were averaged to obtain theaverage fiber length of the mat. FIG. 3 is a SEM photograph used tomeasure the average fiber length of inorganic fibers of the short-fibermat 84. To measure the average fiber length of the long-fiber mat 82,sample mats (each having a size of 10 cm×10 cm) were prepared and tenareas on each mat were randomly selected. Fibers were sampled from theten areas and the sampled fibers were photographed using an opticalmicroscope at 10× magnification. The lengths of at least 50 fibers weremeasured for each of the ten areas. Then, the measured lengths of fibersfrom all of the ten areas were averaged to obtain the average fiberlength of the mat.

The average fiber length of inorganic fibers of the long-fiber mat 82 ispreferably between about 20 mm and about 120 mm, more preferably betweenabout 30 mm and about 70 mm, and still more preferably between about 40mm and about 60 mm. The average fiber length of inorganic fibers of theshort-fiber mat 84 is preferably between about 0.5 mm and about 10 mm,more preferably between about 1 mm and about 5 mm, and still morepreferably between about 2 mm and about 4 mm. Also, the average fiberlength of inorganic fibers of the long-fiber mat 82 is preferably about6 or more times larger than that of inorganic fibers of the short-fibermat 84, and more preferably about 10 or more times larger.

The thickness of a laminated sheet according to an embodiment of thepresent invention is preferably, but not limited to, between about 6 mmand about 20 mm. Normally, a thin sheet (for example, having a thicknesssmaller than about 6 mm) is less likely to develop cracks. Therefore,when the thickness is small, a single layer sheet made of a long-fibermat may be used instead of a laminated sheet according to an embodimentof the present invention. The density of a laminated sheet according toan embodiment of the present invention is preferably, but not limitedto, between about 0.15 g/cm³ and about 0.30 g/cm³. The grammage of alaminated sheet according to an embodiment of the present invention ispreferably, but not limited to, between about 500 g/m² and about 3000g/m². “Grammage” in this case indicates the gram weight of fibers perunit area of a laminated sheet. Also, when a binder is contained in alaminated sheet, “grammage” may indicate the gram weight of fibers andbinder per unit area of the laminated sheet.

In this embodiment, the thicknesses, densities, and grammages of along-fiber mat and a short-fiber mat constituting a laminated sheet arenot limited to specific values. Also in this embodiment, the ratio ofthe thickness of a long-fiber mat to the thickness of a short-fiber matis preferably, but not limited to, between about 2:8 and about 5:5.

As described above, the laminated sheet 30 of this embodiment isproduced by laminating the long-fiber mat 82 and the short-fiber mat 84.To improve the adhesion strength between the long-fiber mat 82 and theshort-fiber mat 84, an interface layer may be provided at the interfacebetween the two mats. In the present application, an “interface layer”indicates any layer that is provided at the interface between twoadjacent layers and that is different in composition from the adjacentlayers. Examples of interface layers include an adhesive layerintentionally formed on one or both of the surfaces of adjacent layersto bond them together and a third layer 86 (see FIG. 6) that isspontaneously formed at the interface between adjacent layers as aresult of concentration of, for example, a binder during the productionprocess of a laminated sheet.

The laminated sheet 30 is wound around the exhaust gas processing unit20 such that the long-fiber mat 82 faces outward (i.e. to face thecasing 12) and fixed to the exhaust gas processing unit 20 by fittingthe protrusion 50 and the recess 60 together. The exhaust gas processingunit 20 wrapped by the laminated sheet 30 is fitted into the casing 12by a press-fit method, a clamshell method, a winding/fastening method,or a sizing method to form the exhaust gas processing device 10. Themethods of fitting the exhaust gas processing unit 20 into the casing 12are described later in detail.

FIG. 4 is a drawing illustrating an exemplary configuration of theexhaust gas processing device 10. In this example, the exhaust gasprocessing unit 20 is implemented as a catalyst carrier having multiplethrough-holes extending parallel to the direction of gas flow. Thecatalyst carrier, for example, is composed of porous silicon carbide andhas a honeycomb structure. However, the exhaust gas processing device 10may also be configured otherwise. For example, the exhaust gasprocessing unit 20 may be implemented as a DPF with the ends of itsthrough-holes occluded in a checkerboard pattern. With the exhaust gasprocessing device 10 configured as described above, development ofcracks in the outer surface of the laminated sheet 30 wound around theexhaust gas processing unit 20 can be effectively prevented. In otherwords, the above configuration prevents unprocessed exhaust gas fromleaking through cracks in a sheet out of an exhaust gas processingdevice.

An exemplary process of producing a laminated sheet according to anembodiment of the present invention is described below. FIG. 5 is aflowchart showing an exemplary process of producing a laminated sheetaccording to an embodiment of the present invention.

As shown in FIG. 5, the exemplary process includes the steps ofpreparing a first mat composed of first inorganic fibers (step S110) andlaminating a second mat composed of second inorganic fibers on the firstmat (step S120).

In the descriptions below, a long-fiber mat is prepared in step S110 anda short-fiber mat is laminated on the long-fiber mat in step S120 toproduce a laminated sheet of this embodiment. However, a laminated sheetof this embodiment may also be produced by preparing a short-fiber matin step S110 and then laminating a long-fiber mat on the short-fiber matin step S120.

The long-fiber mat may be produced by a needling method. In thisembodiment, a “needling method” indicates a method of producing a matwhich method includes a step of moving a fiber-interweaving tool such asa needle in and out of a mat. The needling method is described later inmore detail. On the other hand, the short-fiber mat may be produced by asheet making method. In this embodiment, a “sheet making method”indicates a method of making a mat which method includes steps of fiberopening, slurrying, and press-drying. The sheet making method isdescribed later in more detail.

There are roughly two methods for laminating the long-fiber mat and theshort-fiber mat.

One of the methods is called an “indirect lamination” method where along-fiber mat and a short-fiber mat are produced separately and themats are bonded together at the lamination interface to produce alaminated sheet. In the indirect lamination method, the long-fiber matand the short-fiber mat are bonded at the interface, for example, byusing an adhesive, by sewing, or by vacuum pressure bonding (where themats are stacked, the stacked mats are put in a closed container, andthe closed container is evacuated). As an adhesive, an acrylic adhesiveor an acrylate latex may be used. The thickness of an adhesive layer ispreferably, but is not limited to, between about 0.05 mm and about 2 mm.As described above, the adhesive layer may also be called an interfacelayer.

The other one of the methods is called a “direct lamination” methodwhere a short-fiber mat is formed directly on a long-fiber mat and alaminated sheet composed of the two mats is thereby produced. It is alsopossible to prepare a short-fiber mat first and to form a long-fiber matdirectly on the short-fiber mat. The direct lamination method eliminatesthe need to prepare two mats separately and thereby makes it possible tosimplify the production process.

An exemplary process of producing a laminated sheet according to anembodiment of the present invention is described below in more detail.

(Preparation of Long-Fiber Mat)

As described above, a long-fiber mat may be produced by a needlingmethod. In the exemplary process of producing a long-fiber mat below, amixture of alumina and silica is used as the material of inorganicfibers for the long-fiber mat. However, the material of inorganic fibersis not limited to a mixture of alumina and silica. For example,inorganic fibers for the long-fiber mat may be made of alumina or silicaalone. Also, other materials may be used to produce inorganic fibers forthe long-fiber mat.

A precursor of inorganic fibers is prepared by adding silica sol to abasic aluminum chloride solution (aluminum content: 70 g/l, Al/Cl=1.8[atomic ratio]) so that the proportion of aluminum to silica becomes,for example, about 60-80 to about 40-20. The proportion of aluminum tosilica is more preferably about 70-74 to about 30-26. If the percentageof alumina is about 60% or lower, the percentage of mullite generatedfrom alumina and silica becomes low. A low percentage of mullite resultsin high thermal conductivity of the produced long-fiber mat andtherefore reduces the thermal insulation performance of the long-fibermat.

Next, an organic polymer of, for example, polyvinyl alcohol is added tothe precursor of alumina fibers. The resulting liquid is concentratedand a spinning solution is thereby prepared. Then, the spinning solutionis spun by a blowing method.

In the blowing method, fibers are formed by using streams of air jettedout from air nozzles and streams of the spinning solution ejected fromspinning solution supplying nozzles. The gas flow speed from each slitof the air nozzles is preferably about 40 m/s to about 200 m/s. Thediameter of each of the spinning solution supplying nozzles ispreferably between about 0.1 mm and about 0.5 mm and the flow rate ofthe spinning solution from each of the spinning solution supplyingnozzles is preferably between about 1 ml/h and about 120 ml/h and morepreferably between about 3 ml/h and about 50 ml/h. With the aboveconditions, the spinning solution ejected from the spinning solutionsupplying nozzles does not form a mist but sufficiently stretches in theform of fibers and the formed fibers do not easily adhere to each other.Thus, uniform alumina fibers with a narrow fiber diameter distributioncan be prepared by optimizing the spinning conditions.

The average diameter of the inorganic fibers is preferably, but notlimited to, between about 3 μm and about 10 μm.

The average diameter of the inorganic fibers is measured as describedbelow. The alumina fibers prepared as described above are put in acylinder and compression-milled at 20.6 MPa to prepare a sample. Thesample is put on a sieve and a portion of the sample passing through thesieve is used as a specimen for electron microscopic observation. Gold,for example, is deposited on the specimen and the specimen isphotographed using an electron microscope at about 1500× magnification.Using the photograph, the diameters of at least 40 fibers are measured.The above steps are repeated for five specimens and the measured valuesare averaged to obtain the average diameter of the inorganic fibers.

The obtained alumina fibers are stacked to prepare a raw sheet. Then, aneedling step is performed on the raw sheet. Normally, a needling deviceis used for this step.

A needling device includes a needle board movable back and forth in theneedling direction (for example, in the vertical direction) and a pairof support plates to be placed on the upper and lower surfaces of theraw sheet. On the needle board, a large number of needles for piercingthe raw sheet are arranged, for example, at a density of about 25 toabout 5000 needles per 100 cm². Each of the support plates has a largenumber of through-holes corresponding to the needles. The raw sheet issandwiched between the support plates and the needles are caused to movein and out of the raw sheet by moving the needle board toward and awayfrom the raw sheet. As a result, a large number of interwoven pointswhere the alumina fibers are interwoven with each other are formed. Theneedling device may also include a conveying unit for conveying the rawsheet in a direction (that is substantially parallel to the upper andlower surfaces of the raw sheet) at a uniform conveying speed (forexample, at about 20 mm/sec). The conveying unit enables forwarding theraw sheet while needling the raw sheet and thereby eliminates the needto manually forward the raw sheet each time after the needle board ismoved back and forth.

As an alternative configuration, a needling device may include twoneedle boards. In this case, a support plate is provided for each of theneedle boards. The raw sheet is sandwiched between the support platesand the needle boards are placed, respectively, above and below the rawsheet. Needles on one of the needle boards are arranged so as not tooverlap with needles on the other one of the needle boards. Each of thesupport plates has through-holes corresponding to the needles of both ofthe needle boards so that the needles do not touch the support plateswhen needling the raw sheet. With the needling device having thealternative configuration, the raw sheet is sandwiched by the supportplates and needled from both sides by the two needling boards. Thus, theneedling device having the alternative configuration makes it possibleto reduce the time needed for a needling step. Also, the alternativeconfiguration makes it possible to reduce the number of needles on oneneedle board while increasing the total number of needles.

At the interwoven points formed by the needling step, the interwovenfibers are oriented in the direction the fibers are stacked. Thisstructure increases the strength in the stack direction of the rawsheet.

After the needling step, the raw sheet is heated from the ambienttemperature and calcined for about 0.5 to about 2 hours at a maximumtemperature of about 1250° C. to produce a long-fiber mat.

The long-fiber mat, if needed, may be impregnated with a binder such asan organic resin. This impregnation step makes it possible to reduce thebulk of the long-fiber mat and to prevent inorganic fibers from fallingoff the long-fiber mat. The impregnation step may also be performed at alater stage. For example, when a laminated sheet is produced by theindirect lamination method, the impregnation step may be performed afterthe long-fiber mat and the short-fiber mat are bonded. In this case, theimpregnation step can be performed from either side of the producedlaminated sheet When a laminated sheet is produced by the directlamination method, the long-fiber mat can be impregnated with a binder,as described later, during a step of forming the short-fiber mat on thelong-fiber mat. Therefore, in this case, a separate impregnation step isnot necessary. Also, even in the direct lamination method, theimpregnation step may be performed after a laminated sheet is produced.

In the impregnation step, the amount of binder in an impregnated mat orlaminated sheet is preferably between about 1.0 wt % and about 10.0 wt%. When the amount of binder is about 1.0 wt % or more, the binder cansufficiently prevent the inorganic fibers from falling off. When theamount of binder is 10.0 wt % or less, the amount of organic componentsin exhaust gas emitted from an exhaust gas processing device does notincrease.

As a binder, an organic binder such as an epoxy resin, an acrylic resin,a gum resin, or a styrene resin is preferable. For example, acrylicrubber (ACM), acrylonitrile-butadiene rubber (NBR), and styrenebutadiene rubber (SBR) may be used.

To impregnate a long-fiber mat with a binder, an aqueous dispersion ofthe binder is sprayed onto the long-fiber mat. The excess solid contentand water added to the long-fiber mat in the impregnation step areremoved as described below.

The excess solid content may be removed by suction using a suctiondevice such as a vacuum pump. The excess water may be removed by heatingthe long-fiber mat at a temperature between about 90° C. and about 160°C. and/or by compressing the long-fiber mat at a pressure between about40 kPa and about 100 kPa.

A long-fiber mat composed of inorganic fibers having an average fiberlength between about 20 mm and about 120 mm is prepared through theprocess as described above.

(Preparation of Short-Fiber Mat)

As described above, a short-fiber mat may be produced by a sheet makingmethod. In the exemplary process of producing a short-fiber mat below, amixture of alumina and silica is used as the material of inorganicfibers for the short-fiber mat. However, the material of inorganicfibers is not limited to a mixture of alumina and silica. For example,inorganic fibers for the short-fiber mat may be made from alumina orsilica alone. Also, other materials may be used to produce inorganicfibers for the short-fiber mat.

First, a fiber opening step is performed.

The fiber opening step may be composed of a dry opening step alone or adry opening step and a wet opening step. In the dry opening step,inorganic fibers prepared as described in the exemplary process ofproducing a long-fiber mat are opened using, for example, a feathermill. In the wet opening step, flocculent fibers obtained by the dryopening step are put into a wet opening device and opened further. As awet opening device, a pulper or a mixer may be used. Through the abovefiber opening step, opened raw fibers are obtained.

Next, about 750 g of the obtained raw fibers and about 75 kg of waterare put into an agitator and agitated for about one to about fiveminutes. About 4 wt % to about 8 wt % of an organic binder is added tothe resulting liquid and the liquid is agitated for about one to aboutfive minutes. Then, about 0.5 wt % to about 1.0 wt % of an inorganicbinder is added to the liquid and the liquid is agitated for about oneto about five minutes. Further, about 0.5 wt % of a flocculant is addedto the liquid and the liquid is agitated for about two minutes toprepare slurry.

As the inorganic binder, for example, alumina sol and/or silica sol maybe used. As the organic binder, for example, a rubber-based material, awater-soluble organic high polymer, a thermoplastic resin, or athermo-setting resin may be used. As the flocculant, for example,PERCOL(R) 292 (Ciba Specialty Chemicals) may be used.

The slurry is poured into a mold to form short-fiber mat material andthe short-fiber mat material is dehydrated. A filtration mesh (meshcount: 30) may be provided at the bottom of the mold to allow thedrainage of water in the slurry. With such a mold, it is possible toperform molding and dehydration of a short-fiber mat material at thesame time. Also, water in the short-fiber mat material may be suctionedvia the filtration mesh using, for example, a suction pump or a vacuumpump.

Then, the short-fiber mat material is taken out of the mold, pressed bya pressing machine to a thickness about 0.3 to about 0.5 times as largeas the original thickness, and heated and dried, for example, at about90° C. to about 150° C. for about five minutes to about one hour toproduce a short-fiber mat.

The produced short-fiber mat may be impregnated at this stage with abinder as described above. Alternatively, when a laminated sheet isproduced by the indirect lamination method, the impregnation step may beperformed after the short-fiber mat and the long-fiber mat are bonded.

A short-fiber mat composed of inorganic fibers having an average fiberlength between about 0.5 mm and about 10 mm is prepared through theprocess as described above.

(Combining Short-Fiber Mat and Long-Fiber Mat)

The short-fiber mat and the long-fiber mat prepared as described aboveare combined to produce a laminated sheet according to an embodiment ofthe present invention.

To combine the mats, the indirect lamination method or the directlamination method may be used. In the direct lamination method, aprepared long-fiber mat is placed on the bottom of the mold used in theabove exemplary process and a short-fiber mat is formed on thelong-fiber mat. Thus, with the direct lamination method, the long-fibermat and the short-fiber mat are combined in the short-fiber matpreparation process. Compared with a method where the long-fiber mat andthe short-fiber mat are prepared separately, the direct laminationmethod makes it possible to simplify the laminated sheet productionprocess.

One possible problem with the direct lamination method is that theadhesion strength at the interface between the long-fiber mat and theshort-fiber mat of a produced laminated sheet may become low. However,according to test results, the adhesion strength at the interfacebetween the long-fiber mat and the short-fiber mat of a laminated sheetproduced by the direct lamination method was as excellent as that of alaminated sheet produced by the indirect lamination method. As shown inFIG. 6, formation of a third layer 86 (an interface layer) is observedat the interface between the long-fiber mat 82 and the short-fiber mat84 of the laminated sheet 30 produced by the direct lamination method.It is presumed that the binder added to the slurry when preparing theshort-fiber mat 84 concentrates at the interface and forms the thirdlayer 86. It is also presumed that the third layer 86 functions as anadhesive layer and thereby increases the adhesion strength at theinterface between the long-fiber mat 82 and the short-fiber mat 84. Thethickness of the third layer 86, although varying depending on theproduction conditions of the laminated sheet 30, is between about 0.05mm and about 2 mm.

FIG. 7 is a flowchart showing an exemplary process of producing anexhaust gas processing device using a laminated sheet according to anembodiment of the present invention. In step S210, a laminated sheet isprepared by laminating a long-fiber mat and a short-fiber mat asdescribed above. In step S220, the prepared laminated sheet is woundaround an exhaust gas processing unit such that the long-fiber mat facesoutward. Then, in step S230, the exhaust gas processing unit wrapped bythe laminated sheet is fitted into a casing by a press-fit method, aclamshell method, a winding/fastening method, or a sizing method to forman exhaust gas processing device.

The methods of fitting the exhaust gas processing unit into the casingare described below with reference to the accompanying drawings. FIGS.8, 9, 10, and 11 are drawings illustrating the methods of fitting theexhaust gas processing unit 20 wrapped by the holding sealer 24(hereafter called a wrapped exhaust gas processing unit 210) into acasing.

In the press-fit method, the wrapped exhaust gas processing unit 210 ispressed into a casing 121 through one of its openings to produce theexhaust gas processing device 10. To make it easier to insert thewrapped exhaust gas processing unit 210 into the casing 121, apress-fitting jig 230 may be used. As shown in FIG. 8, the innerdiameter of the press-fitting jig 230 decreases gradually from one endto the other and the minimum inner diameter of the press-fitting jig 230is substantially the same as the inner diameter of the casing 121. Thewrapped exhaust gas processing unit 210 is inserted into thepress-fitting jig 230 through its wider opening and pressed into thecasing 121 through the narrower opening of the press-fitting jig 230.

In the clamshell method, a casing made up of multiple casing parts isused. In FIG. 9, a casing 122 is made up of opposing casing parts 122Aand 122B. The wrapped exhaust gas processing unit 210 is placed in afirst one of the casing parts 122A and 122B and a second one of thecasing parts 122A and 122B is coupled to the first one to produce theexhaust gas processing device 10. The casing parts 122A and 122B arejoined, for example, by welding flanges 220 (220A and 220B) together.

In the winding/fastening method, as shown in FIG. 10, a metal plate(casing) 123 to be used as a casing is wound around the wrapped exhaustgas processing unit 210 and tied and pressed, for example, with a wirerope onto the outer surface of the wrapped exhaust gas processing unit210 to produce a predetermined contact pressure. Then, the ends of themetal plate 123 are welded together to produce the exhaust gasprocessing device 10 where the wrapped exhaust gas processing unit 210is fitted in the casing 123.

In the sizing method, as shown in FIG. 11, the wrapped exhaust gasprocessing unit 210 is inserted into a metal shell 124 having an innerdiameter that is larger than the outer diameter of the wrapped exhaustgas processing unit 210. Then, the outer surface of the metal shell 124is uniformly pressed, for example, by a press machine to reduce the sizeof the metal shell 124 (sizing: JIS Z2500-4002). This sizing step makesit possible to accurately adjust the inner diameter of the metal shell124 to fit the wrapped exhaust gas processing unit 210.

The material of a casing used in the above fitting methods is preferablya metal such as a heat-resistant alloy.

As described above, a laminated sheet according to an embodiment of thepresent invention is made up of a first mat that is stronger andcomposed of inorganic fibers having a larger average fiber length and asecond mat composed of inorganic fibers having a shorter average fiberlength. The laminated sheet is wound around an exhaust gas processingunit such that the first mat faces outward. This configuration reducesthe influence of the difference between the outer and innercircumferences of the laminated sheet and thereby reduces development ofcracks in the outer surface of the laminated sheet even when itsthickness is large. Also, unlike a conventional sheet having grooves forreducing the influence of the difference between outer and innercircumferences, a laminated sheet according to an embodiment of thepresent invention can be used for exhaust gas processing units havingdifferent shapes and dimensions without changing its configuration andtherefore can be produced efficiently.

In the above embodiments, a laminated sheet is composed of two mats: along-fiber mat and a short-fiber mat. However, a laminated sheetaccording to the present invention may also be made up of three or moremats as long as the outermost mat, which faces outward when wound aroundan exhaust gas processing unit, of the laminated sheet is composed offibers having the average fiber length larger than the average fiberlengths of other mats. Also, another embodiment of the present inventionprovides an exhaust gas processing device including such a laminatedsheet as a holding sealer.

To evaluate the advantageous effects of the present invention, sheetswere produced and tests were performed on the produced sheets asdescribed below.

EXAMPLE 1

A laminated sheet of example 1 was produced as described below.

First, a long-fiber mat was produced. A precursor of alumina fibers(Al₂O₃:SiO₂=72:28) was prepared by adding silica sol to a basic aluminumchloride solution (aluminum content: 70 g/l, Al/Cl=1.8 [atomic ratio]).Next, an organic polymer of polyvinyl alcohol was added to the precursorof alumina fibers. The resulting liquid was concentrated to form aspinning solution and the spinning solution was spun by a blowingmethod. The obtained alumina fibers were folded and stacked to prepare araw sheet of alumina fibers. Then, a needling step was performed on theraw sheet. The needling step was performed from both sides of the rawsheet using a pair of needle boards placed above and below the rawsheet, each of the needle boards having needles arranged at a density of50 needles per 100 cm². The resulting raw sheet had interwoven points ata density of 1/cm².

After the needling step, the raw sheet was heated from the ambienttemperature and calcined for one hour at a maximum temperature of 1250°C. to produce a long-fiber mat with a grammage of 950 g/m² and athickness of 4 mm. The average fiber length of the alumina fibers wasabout 50 mm. The average diameter of the alumina fibers was 6.2 μm andthe minimum diameter was 3.2 μm.

Then, a short-fiber mat was formed directly on the prepared long-fibermat as described below.

First, opened inorganic fibers were prepared as raw fibers. In thisexample, alumina fibers (hereafter called “cotton bulk”) composed ofalumina and silica (mixture ratio 72:28) were used.

Next, 790 g of the cotton bulk was mixed with 79 kg of water and themixture was agitated for five minutes using an agitator. Then, 39.5 g ofan organic binder (latex) was added to the resulting liquid and theliquid was agitated for five minutes. After that, 7.9 g of an inorganicbinder (alumina sol) was added to the liquid and the liquid was agitatedfor five minutes. Further, 3.95 g of a flocculant (PERCOL® 292) wasadded to the liquid and the liquid was agitated for one minute toprepare slurry.

In the next step, the prepared long-fiber mat was placed on the bottom,which is formed as a filtration mesh (mesh count: 30), of a mold (930 mm(length)×515 mm (width)×400 mm (depth)). The dimensions of thelong-fiber mat were 930 mm (length)×515 mm (width)×4 mm (thickness). Theprepared slurry was poured onto the long-fiber mat and dehydrated toform a short-fiber mat. In the dehydration step, water in the slurry wassuctioned via the filtration mesh at the bottom of the mold using asuction pump. The stacked long-fiber and short-fiber mats were taken outof the mold and press-dried for 30 minutes at 120° C. and 70 kPa.Through the above steps, a short-fiber mat having a grammage of 1750g/m² and a thickness of 9 mm was formed on the long-fiber mat. Theaverage fiber length of the alumina fibers in the short-fiber mat wasabout 3 mm.

Thus, in example 1, a laminated sheet with a thickness of 13 mm and adensity of 0.21 g/cm³ was prepared.

EXAMPLE 2

First, a long-fiber mat was prepared in substantially the same manner asin example 1, except that the amount of stacked alumina fibers waschanged. The grammage of the prepared long-fiber mat was 1350 g/m² andthe thickness was 6 mm. The average fiber length of the alumina fiberswas about 50 mm. The average diameter of the alumina fibers was 6.2 μmand the minimum diameter was 3.2 μm.

Next, 610 g of cotton bulk made of alumina fibers composed of aluminaand silica (mixture ratio 72:28) was mixed with 61 kg of water and themixture was agitated for five minutes using an agitator. Then, 30.5 g ofan organic binder (latex) was added to the resulting liquid and theliquid was agitated for five minutes. After that, 6.1 g of an inorganicbinder (alumina sol) was added to the liquid and the liquid was agitatedfor five minutes. Further, 3.05 g of a flocculant (PERCOL® 292) wasadded to the liquid and the liquid was agitated for one minute toprepare slurry.

In the next step, the prepared long-fiber mat (with a grammage of 1350g/m²) was placed on the bottom, which is formed as a filtration mesh(mesh count: 30), of a mold (930 mm (length)×515 mm (width)×400 mm(depth)). The dimensions of the long-fiber mat were 930 mm (length)×515mm (width)×6 mm (thickness). The prepared slurry was poured onto thelong-fiber mat and dehydrated to form a short-fiber mat. In thedehydration step, water in the slurry was suctioned via the filtrationmesh at the bottom of the mold using a suction pump. The stackedlong-fiber and short-fiber mats were taken out of the mold andpress-dried for 30 minutes at 120° C. and 70 kPa. Through the abovesteps, a short-fiber mat having a grammage of 1350 g/m² and a thicknessof 7 mm was formed on the long-fiber mat. The average fiber length ofthe alumina fibers in the short-fiber mat was about 3 mm.

Thus, in example 2, a laminated sheet with a thickness of 13 mm and adensity of 0.21 g/cm³ was prepared.

EXAMPLE 3

First, a long-fiber mat was prepared in substantially the same manner asin example 1, except that the amount of stacked alumina fibers waschanged. The grammage of the prepared long-fiber mat was 950 g/m² andthe thickness was 4 mm. The average fiber length of the alumina fiberswas about 50 mm. The average diameter of the alumina fibers was 6.2 μmand the minimum diameter was 3.2 μm.

Next, 430 g of cotton bulk made of alumina fibers composed of aluminaand silica (mixture ratio 72:28) was mixed with 43 kg of water and themixture was agitated for five minutes using an agitator. Then, 21.5 g ofan organic binder (latex) was added to the resulting liquid and theliquid was agitated for five minutes. After that, 4.3 g of an inorganicbinder (alumina sol) was added to the liquid and the liquid was agitatedfor five minutes. Further, 2.15 g of a flocculant (PERCOL® 292) wasadded to the liquid and the liquid was agitated for one minute toprepare slurry.

In the next step, the prepared long-fiber mat (with a grammage of 950g/m²) was placed on the bottom, which is formed as a filtration mesh(mesh count: 30), of a mold (930 mm (length)×515 mm (width)×400 mm(depth)). The dimensions of the long-fiber mat were 930 mm (length)×515mm (width)×4 mm (thickness). The prepared slurry was poured onto thelong-fiber mat and dehydrated to form a short-fiber mat. In thedehydration step, water in the slurry was suctioned via the filtrationmesh at the bottom of the mold using a suction pump. The stackedlong-fiber and short-fiber mats were taken out of the mold andpress-dried for 30 minutes at 120° C. and 70 kPa. Through the abovesteps, a short-fiber mat having a grammage of 950 g/m² and a thicknessof 5 mm was formed on the long-fiber mat. The average fiber length ofthe alumina fibers in the short-fiber mat was about 3 mm.

Thus, in example 3, a laminated sheet with a thickness of 9 mm and adensity of 0.21 g/cm³ was prepared.

COMPARATIVE EXAMPLE 1

In comparative example 1, 1220 g of cotton bulk made of alumina fiberscomposed of alumina and silica (mixture ratio 72:28) was mixed with 122kg of water and the mixture was agitated for five minutes using anagitator. Then, 61 g of an organic binder (latex) was added to theresulting liquid and the liquid was agitated for five minutes. Afterthat, 12.2 g of an inorganic binder (alumina sol) was added to theliquid and the liquid was agitated for five minutes. Further, 6.1 g of aflocculant (PERCOL® 292) was added to the liquid and the liquid wasagitated for one minute to prepare slurry.

In the next step, the prepared slurry was poured into a mold (930 mm(length)×515 mm (width)×400 mm (depth)) having a filtration mesh (meshcount: 30) at the bottom and was dehydrated to form a short-fiber mat.In the dehydration step, water in the slurry was suctioned via thefiltration mesh at the bottom of the mold using a suction pump. Then,the short-fiber mat was taken out of the mold and press-dried for 30minutes at 120° C. and 70 kPa. Thus, in comparative example 1, a singlelayer sheet composed only of a short-fiber mat with a grammage of 2700g/m², a thickness of 13 mm, and a density of 0.21 g/cm³ was prepared.The average fiber length of the alumina fibers in the single-layer sheetwas about 3 mm.

COMPARATIVE EXAMPLE 2

In comparative example 2, 860 g of cotton bulk made of alumina fiberscomposed of alumina and silica (mixture ratio 72:28) was mixed with 86kg of water and the mixture was agitated for five minutes using anagitator. Then, 43 g of an organic binder (latex) was added to theresulting liquid and the liquid was agitated for five minutes. Afterthat, 8.6 g of an inorganic binder (alumina sol) was added to the liquidand the liquid was agitated for five minutes. Further, 4.3 g of aflocculant (PERCOL® 292) was added to the liquid and the liquid wasagitated for one minute to prepare slurry.

In the next step, the prepared slurry was poured into a mold (930 mm(length)×515 mm (width)×400 mm (depth)) having a filtration mesh (meshcount: 30) at the bottom and was dehydrated to form a short-fiber mat.In the dehydration step, water in the slurry was suctioned via thefiltration mesh at the bottom of the mold using a suction pump. Then,the short-fiber mat was taken out of the mold and press-dried for 30minutes at 120° C. and 70 kPa. Thus, in comparative example 2, a singlelayer sheet composed only of a short-fiber mat with a grammage of 1900g/m², a thickness of 9 mm, and a density of 0.21 g/cm³ was prepared. Theaverage fiber length of the alumina fibers in the single-layer sheet wasabout 3 mm.

Table 1 shows grammages, thicknesses, and densities of the laminatedsheets of examples 1 through 3 and the single-layer sheets ofcomparative examples 1 and 2. In table 1, grammages are providedseparately for long-fiber mats and short-fiber mats and individualthicknesses of long-fiber mats and short-fiber mats are provided inaddition to the thicknesses of sheets.

TABLE 1 Example/ Long-fiber mat Short-fiber mat Sheet Sheet WindingComparative Grammage Thickness Grammage Thickness thickness density testexample (g/m²) (mm) (g/m²) (mm) (mm) (g/cm³) results Example 1 950 41750 9 13 0.21 ◯ Example 2 1350 6 1350 7 13 0.21 ◯ Example 3 950 4 950 59 0.21 ◯ Comparative 2700 13 13 0.21 X example 1 Comparative 1900 9 90.21 X example 2

<Evaluation Test>

Using the sheets prepared as described above, a winding test wasperformed. In the winding test, each sheet was wound around a cylinderhaving an outer diameter of 5 inches and ends of the sheet were fittedtogether to fix it to the cylinder. Then, development of cracks in theouter surface of each sheet was checked for by eye observation. Each ofthe laminated sheets of examples 1 through 3 was wound around thecylinder such that the long-fiber mat faces outward.

<Test Results>

The results of the winding test are shown in table 1 above. FIG. 12 is aphotograph of the laminated sheet of example 1 used in the winding test.FIG. 13 is a photograph of the single-layer sheet of comparative example1 used in the winding test. As shown in table 1, cracks were notobserved in the outer surfaces of the laminated sheets of examples 1through 3. On the other hand, cracks were observed on the outer surfacesof the single-layer sheets of comparative examples 1 and 2. The testresults indicate that a laminated sheet composed of a long-fiber mat anda short-fiber mat according to an embodiment of the present inventionshows sufficient strength even when the thickness is large.

Laminated sheets according to embodiments of the present invention canbe used as components of exhaust gas processing devices for automobiles.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese Priority Application No.2006-266376, filed on Sep. 29, 2006, the entire contents of which arehereby incorporated herein by reference.

Also, the entire contents of Japanese Patent No. 3072281 and JISZ2500-4002 are hereby incorporated herein by reference.

1. A laminated sheet, comprising: a first mat containing first inorganicfibers; and a second mat containing second inorganic fibers, the secondmat being laminated on the first mat; wherein an average fiber length ofthe first inorganic fibers is larger than an average fiber length of thesecond inorganic fibers.
 2. The laminated sheet as claimed in claim 1,wherein the first inorganic fibers and the second inorganic fibers arecomposed of a same material.
 3. The laminated sheet as claimed in claim1, wherein the average fiber length of the first inorganic fibers iswithin a range between about 20 mm and about 120 mm.
 4. The laminatedsheet as claimed in claim 1, wherein the average fiber length of thesecond inorganic fibers is within a range between about 0.5 mm and about10 mm.
 5. The laminated sheet as claimed in claim 1, wherein the firstmat and/or the second mat further contain a binder.
 6. The laminatedsheet as claimed in claim 1, further comprising: an interface layer atan interface between the first mat and the second mat.
 7. The laminatedsheet as claimed in claim 6, wherein a thickness of the interface layeris within a range between about 0.05 mm and about 2 mm.
 8. The laminatedsheet as claimed in claim 1, wherein a thickness of the laminated sheetis within a range between about 6 mm and about 20 mm.
 9. A method ofproducing a laminated sheet, comprising: a first step of preparing afirst mat containing first inorganic fibers; and a second step oflaminating a second mat containing second inorganic fibers on the firstmat; wherein an average fiber length of the first inorganic fibers islarger than an average fiber length of the second inorganic fibers. 10.The method as claimed in claim 9, wherein the second step includes asubstep of forming the second mat directly on the first mat.
 11. Themethod as claimed in claim 9, wherein the second step includes substepsof separately preparing the second mat and then laminating the first matand the second mat.
 12. The method as claimed in claim 11, wherein thesecond step further includes a substep of joining the first mat and thesecond mat by adhesive bonding and/or by sewing.
 13. The method asclaimed in claim 9, wherein the first mat is prepared by a needlingmethod.
 14. The method as claimed in claim 9, wherein the second mat isprepared by a sheet making method.
 15. An exhaust gas processing device,comprising: an exhaust gas processing unit; and a holding sealer woundaround at least a portion of an outer surface of the exhaust gasprocessing unit; wherein the holding sealer is composed of the laminatedsheet as claimed in claim 1 and wound around the exhaust gas processingunit such that the first mat of the laminated sheet faces outward. 16.The exhaust gas processing device as claimed in claim 15, wherein theexhaust gas processing unit is a catalyst carrier or an exhaust gasfilter.
 17. A method of producing an exhaust gas processing deviceincluding an exhaust gas processing unit and a holding sealer woundaround at least a portion of an outer surface of the exhaust gasprocessing unit, comprising the steps of: preparing the holding sealerwith the laminated sheet produced by the method as claimed in claim 9;and winding the holding sealer around the exhaust gas processing unitsuch that the first mat of the laminated sheet faces outward.
 18. Themethod as claimed in claim 17, wherein the exhaust gas processing unitis a catalyst carrier or an exhaust gas filter.